This proposal is directed toward understanding mechanisms controlling the development of neuronal connections in the larval anuran (Rana catesbeiana). Electrophysiological techniques will be applied to the spinal cord maintained in vitro to assess the development of functional connections onto motoneurons of the lateral motor column (LMC) and, separately, onto motoneurons of the medial motor column. The effect upon the development of LMC motoneurons of removal of the peripheral endorgan and of substitution of ectopic tail muscle (normally innervated by medial motor column cells) will be determined. Concomittantly retrograde filling of motorneurons with horseradish peroxidase (HRP) will be used to correlate motoneuron morphology with the development of reflex participation. A further aim of this proposal is to assess the changes in the connections among fibers of brainstem origin and spinal neurons as the tadpole metamorphoses from an aquatic to a terrestrial animal. This study involves comparisons of the sources of descending fibers in the tadpole and the juvenile frog. To determine whether the same brainstem neurons project to the spinal cord in tadpole and frog, a technique of dual sequential labeling with HRP in the tadpole followed by labeling with radioactive wheat-germ aglutinin in the same animal after metamorphosis is proposed.